1. Field of the Invention
The present invention relates to an apparatus for controlling a load applied to a motor as a power supply of an electric vehicle.
2. Description of Related Art
Recently, environmental pollution has become a serious problem to be solved, and effective measures for purifying emission gases from gasoline or diesel-powered vehicles have been required. Under these circumstances, electric vehicles which are free from emission gases have been lately put to practical use for specific purposes. However, electric vehicles are still considerably inferior to the gasoline or diesel-powered vehicles in continuously travelable distance and acceleration performance. In order to ensure running ability identical to that of gasoline or diesel-powered vehicles, electric vehicles are required to obtain the travelable distance on one battery charge identical or near that of the gasoline or diesel-powered vehicles on one oil supply. And to answer the above demand, various researches have been made.
An electric vehicle disclosed in Japanese patent application laid-open No. Hei 4-340301 includes a chopper between a motor and a battery, and capacitors on inlet and outlet sides of the chopper. With this arrangement, kinetic and potential energy due to braking is converted to electric energy, and is sent back to the battery.
FIG. 11 illustrates a circuit diagram of a power supply device for the electric vehicle disclosed in the above publication. As shown, between a DC motor M and a battery Ba, a chopper composed of a reactor L, a switching element Q2 which is turned on and off by a control mechanism, and a diode D2 which supplies an electromotive force of the motor M to the battery Ba when the element Q2 is turned off from on. Capacitors C1 and C2 are respectively connected to both sides of the chopper for blocking high frequency wave. When the motor M is decelerated, an electric current due to the electromotive force of the motor M flows through the reactor L and the element which is in on state. When the element Q2 is in the off state, the above electric current flows to the diode D2, and is sent back to the battery Ba and collected thereby at a high efficiency by virtue of the reactor L.
A hybrid power supply apparatus for an electric vehicle disclosed in Japanese Patent application laid-open No. Hei 5-30608 is schematically illustrated in FIG. 12. As shown, the hybrid power supply apparatus includes an electric current control circuit 210 which controls the amount of current flow in response to the deceleration amount. The circuit 210 is connected between a capacitor 110, a battery 120 and a convertor 140. The apparatus further includes a collecting and charging circuit composed of a switch SW2 which is connected to the battery 120 and a current control circuit 130, and a discharging circuit composed of a switch SW1 and a diode D. With this arrangement, the burden imposed on the battery 120 upon discharge and collection of the energy due to acceleration and deceleration of electric vehicles is shared with the capacitor 110, and accordingly, the rapid charge of the battery due to deceleration of the electric vehicles is restrained to prevent deterioration of the battery and to prolong the battery life. Furthermore, a large amount of energy during deceleration of the vehicles is sent back to the capacitor 110 and is later used as energy for acceleration of vehicles. So, the burden of the battery during acceleration of vehicles is lightened, whereby the utilization efficiency of the battery is improved, and accordingly, the travelable distance of the electric vehicles is increased and the acceleration and deceleration performances are both improved.
The power supply apparatus disclosed in Japanese patent application laid-open No. Hei 4-340301, however, has the following problem. When electric vehicles are accelerated and decelerated while running, the amount of electric current flowing from the battery varies. Especially when the electric vehicles are rapidly accelerated, the battery current drastically varies, thereby remarkably increasing current pulsations of the battery.
Moreover, the hybrid power supply apparatus disclosed in Japanese Patent application laid-open No. Hei 5-30608 also has the following problem. A capacitor of high capacitance and the battery are connected in parallel without any means of controlling the ratio of the power to be supplied to the motor from the capacitor to that to be supplied to the motor from the battery. Therefore, when the electric vehicles are rapidly accelerated, there is a possibility that a large amount of current flows from not only the capacitor but also the battery, and in such a case, the battery current greatly varies to increase current pulsations of the battery.
Assuming that the battery volume E (t) is constant, the internal loss of the battery is expressed by: EQU loss=.intg.I(t).sup.2 R d t
wherein I (t) is electric current and R is internal resistance of the battery.
Namely, the internal loss is in proportion to the square of the electric current of the battery. This equation shows that when an equal energy is supplied within a given period of time, the internal loss of the battery increases in the case of large current pulsations, as compared with the case of small current pulsations. The internal loss of the battery can be decreased by reducing the current pulsations thereof. In the case of batteries of the identical capacitance, the dischargeable capacitance thereof can be increased by restraining the current pulsations thereof.
FIGS. 13 through 15 are circuit diagrams of conventional power supply apparatuses for electric vehicles. FIGS. 16 through 18 show variations of battery current with time in the running mode of LA#4. The running mode of LA#4 means a general running pattern for use in measurement of the electric vehicles. Point A denotes the electric current when the electric vehicle is rapidly accelerated.
FIG. 13 illustrates the circuit diagram of the power supply apparatus disclosed in Japanese Patent application No. Hei 4-340301, of which the capacitor C1 is of a small capacitance. With this arrangement, the electric current to be supplied to a motor M from a battery E (120 V) is controlled by a chopper composed of the capacitor C1 (5 mF), a motor drive switch, a motor brake switch, a diode, and a coil. FIG. 14 illustrates a power supply apparatus of which the capacitor C1 is of a large capacitance (30 F). In the power supply apparatus of FIG. 15, a diode is further added to the apparatus of FIG. 14 for preventing the collection of electric current by the battery. In FIG. 16, current pulsations are entirely on a large scale. At the point A, an electric current of about 220 A flows. In FIG. 17, peaks of current pulsations of the battery are reduced, as compared with those of FIG. 16. However, upon rapid acceleration, a large amount of current of about 150 A flows. In FIG. 18, the waveform of positive current is identical to that of FIG. 17, however, the entire current pulsations are reduced, since the diode prevents the battery from collecting the current. Similarly to the case of FIG. 17, upon rapid acceleration, a large amount of current of about 150 A flows. The above results show that the capacitor of a high capacitance enables the increase of dischargeable capacitance, but even in such a case, there is a possibility that a large amount of current flows from the battery, whereby internal loss of the battery cannot be perfectly prevented.
In the hybrid power supply apparatus disclosed in Japanese Patent application laid-open No. Hei 5-30608, there is a possibility that a leakage current flows from the capacitor of a large capacitance while an electric vehicle is parked for a long period of time. This causes the electrical potential of the capacitor of a large capacitance to be decreased, and a large amount of current to flow from the battery to the capacitor of a large capacitance at an engine-starting time of an electric vehicle, which obstructs effective use of energy from the battery.
In addition, in order to further decrease the internal loss of the battery for increasing the dischargeable capacitance thereof, the present inventors have contemplated to connect motor control means to the battery directly or to change over it to connect to the chopper and the capacitor of a large capacitance in accordance with the running state of the electric vehicle. With this arrangement, however, the capacitor of a large capacitance does not operate while the electric vehicle is continuously running on an upslope or running in the power mode, which causes the frequency in use of the battery to be increased. As a result, there is a limit to the increase of the travelable distance of an electric vehicle on one battery charge.
Furthermore, when the current pulsations of the battery are on a large scale, the dischargeable capacitance thereof decreases. Accordingly, in this case, if an electric current is supplied from the battery in response to the variation of a load, the battery discharge proceeds, which requires the battery charge in a short period of time.